6066 (1960-1966) GMC Truck Club Bitcoin now accepted here! 
Pay Dues
Pay Dues or become a Site Supporter
 



Go Back   6066 (1960-1966) GMC Truck Club > 6066 GMC Truck Club Forum > GMC V6 and V12 Engines


GMC V6 and V12 Engines Engine repair and rebuilding

Reply
 
Thread Tools
  #1  
Old February 20th, 2015, 03:55 PM
patrickk1 patrickk1 is offline
-= Dues Paid =-
 
Join Date: Feb 2015
Location: Billerica, Ma
Truck: 1963 GMC 1/2 ton
Age: 41
Posts: 12
Rep Power: 60
patrickk1 is on a distinguished road
Default 305 Intake 4bbl Carb Mod

Does anybody have any pictures of an intake modified for a 4bbl carb? I saw reference to it in the archived section of the forum and there's no photos there since its the archive. I see no reference to a 4bbl carb since then in the forums.

"I have converted several GMC V6 Magnum manifolds from 2bbl to a 4bbl, it's really a simple conversion that involves machining the base down flat then cutting out a square bore pattern followed by drilling & tapping 4 new mounting holes and bolting on a new 4bbl base plate which nothing more than a square bore 3/4 inch carb spacer. "No welding is required" in this operation, I can send you some pics tomorrow when I get home.
Pete Chronis Northern Cal."

Thanks
Reply With Quote
  #2  
Old February 20th, 2015, 09:02 PM
bigblockv6 bigblockv6 is offline
Senior Member
 
Join Date: Jan 2013
Location: Rohnert Park, ca.
Truck: 1968 KM2500
Posts: 1,325
Rep Power: 415
bigblockv6 will become famous soon enoughbigblockv6 will become famous soon enough
Default Re: 305 Intake 4bbl Carb Mod

Ok, here's the photo of the manifold.
Attached Images
File Type: jpg 4bbl manifold for GMC V6.jpg (414.8 KB, Multiple views, 45 clicks)
Reply With Quote
  #3  
Old February 20th, 2015, 09:23 PM
patrickk1 patrickk1 is offline
-= Dues Paid =-
 
Join Date: Feb 2015
Location: Billerica, Ma
Truck: 1963 GMC 1/2 ton
Age: 41
Posts: 12
Rep Power: 60
patrickk1 is on a distinguished road
Default Re: 305 Intake 4bbl Carb Mod

Awesome! Thank you.
Reply With Quote
  #4  
Old February 20th, 2015, 09:24 PM
bigblockv6 bigblockv6 is offline
Senior Member
 
Join Date: Jan 2013
Location: Rohnert Park, ca.
Truck: 1968 KM2500
Posts: 1,325
Rep Power: 415
bigblockv6 will become famous soon enoughbigblockv6 will become famous soon enough
Default Re: 305 Intake 4bbl Carb Mod

One more pic from the top of the manifold.
Attached Images
File Type: jpg top view V6 manifold (2).jpg (417.0 KB, Multiple views, 27 clicks)
Reply With Quote
  #5  
Old February 21st, 2015, 02:30 AM
Clarke Clarke is offline
Senior Member
 
Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
Posts: 217
Rep Power: 128
Clarke is on a distinguished road
Default 2bbl versus 4bbl

So, a while back I told you guys I took my truck to the local dyno shop and they said I could use larger jets on my stock 2bbl carb. I only bought the next oversized jets, because I read on this site that other members have actually installed smaller jets (or maybe one jet?). The dyno guys installed the larger jets, ran the truck on their machine, and said I could go up another size larger, but I didn't have anything larger.

Anyway, help me understand the differnce between putting larger jets in a 2bbl versus installing a 4bbl. I'm assuming the 4bbl allows more air to cycle through; therefore, you can mix more gas?

I don't have any desire to pursue the 4bbl mod, just curious what the difference is compared to rejetting the 2bbl. I may buy the next larger size jets if I take it back to the dyno shop.

Thanks,

Clarke
Reply With Quote
  #6  
Old February 21st, 2015, 01:43 PM
patrickk1 patrickk1 is offline
-= Dues Paid =-
 
Join Date: Feb 2015
Location: Billerica, Ma
Truck: 1963 GMC 1/2 ton
Age: 41
Posts: 12
Rep Power: 60
patrickk1 is on a distinguished road
Default Re: 305 Intake 4bbl Carb Mod

4bbl carbs have small primaries and large secondaries. If you keep your foot out of it, you can cruise on just the primaries. The secondaries kick in when you put your foot into it (assuming it's not a double pumper). The quadrajet 4bbl has even smaller primaries that most 4bbl carbs. To me, if tuned perfectly, an appropriate sized 4bbl would have a mpg advantage because the smaller primaries would allow a more precise air/fuel ratio at cruise speed but when you hit the gas, more fuel is going to get dumped down the carb from the large secondaries. Some people say 2bbl get better mileage, some people say 4bbls do. But it could be that some 2bbl just die off around 60mph and its a struggle to get to 70, so you rarely travel at that speed with a 2bbl, which would lower mpg due to wind resistance of the vehicle. I am speaking of carbs in general, not specific to the charateristics of how these v6's run.

More air/fuel would get to the engine from the larger secondaries, if they are open. Less air/fuel if you are going through just the primaries.
Reply With Quote
  #7  
Old February 22nd, 2015, 12:37 PM
turbobill turbobill is offline
Member
 
Join Date: Feb 2013
Location: northern NY when not flying
Truck: 1968 HM80 with a 478M and a few engines plus a Toro-Flow
Posts: 98
Rep Power: 93
turbobill is on a distinguished road
Default Re: 2bbl versus 4bbl

Quote:
Originally Posted by Clarke View Post

Anyway, help me understand the differnce between putting larger jets in a 2bbl versus installing a 4bbl. I'm assuming the 4bbl allows more air to cycle through; therefore, you can mix more gas?

I don't have any desire to
Thanks,

Clarke
To understand, lets discuss the basics first.

An engine is an air pump. More air, more fuel, (mixed in the PROPER proportion) more power. The advantage of a 4 barrel carburetor over a two barrel is that the 4 barrel allows the engine to inhale more air at WOT than the two barrel, thus horsepower is increased.

Installing "larger" jets has nothing to do with what type of carburetor this is performed on. In my last paragraph, I mentioned "proper" proportion of air and fuel. This is commonly refered to as the air/fuel ratio.

This is a very technical subject, but instead, I will keep is simple to help you understand. If the air/fuel mixture is to "rich" (more fuel than is needed for proper combustion), fuel ecomony and power suffer, as well as more carbon deposits, oil contamination and spark plug fouling.

If the air fuel ratio is to "lean" (not enough fuel for proper combustion) power and driveability suffer, and the EGT (exhaust gas temperature) can become excessive.

Apparently, the dyno operator found the fuel/air mixture on your engine to be to lean. They may have measured it with an oxygen sensor or EGT probe in the exhaust, or just suspected it was lean by the way the engine behaved on the dyno. After the jet change, the figures probably showed "richening" the air/fuel ratio was improving the numbers (air fuel ratio, EGT, horsepower)the dyno operator was looking at.
Reply With Quote
  #8  
Old February 23rd, 2015, 02:50 AM
Clarke Clarke is offline
Senior Member
 
Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
Posts: 217
Rep Power: 128
Clarke is on a distinguished road
Default Re: 305 Intake 4bbl Carb Mod

Thanks for the education guys...

Yes, I noticed the dyno guys cut a hole and welded a port into my exhaust Y pipe, assuming for their sensor as you described.
Reply With Quote
Reply

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Re: 4bbl intake Archiver Previous Forum Posts 0 June 30th, 2010 09:19 AM
Re: 4bbl intake Archiver Previous Forum Posts 0 June 30th, 2010 04:58 AM
4bbl intake Archiver Previous Forum Posts 0 June 29th, 2010 01:27 AM
4bbl intake manifolds Archiver Previous Forum Posts 0 October 10th, 2001 01:35 AM
Re: 4bbl intake manifolds Archiver Previous Forum Posts 0 October 10th, 2001 01:23 AM


All times are GMT +1. The time now is 06:54 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2013, Jelsoft Enterprises Ltd 671711554703|1710438870|0